In conventional cellular mobile communication systems, a UE (user equipment) has to communicate with another UE only through the relaying of base stations even if the two UEs are very close to each other. FIG. 1 illustrates the conventional communication mode. However, in some cases when the distance between two UEs who camp on the same cell is very close, it can be a more reasonable way for them to communicate directly, rather than through the relaying of base stations. This method is the so-called peer-to-peer communication, abbr. as P2P.
FIG. 2 illustrates the P2P communication between two UEs. Referring to FIG. 2, assume that the two UEs are camping in the same cell and the distance between them satisfies the requirement for establishing P2P connection, the dashed line represents signaling link between the UTRAN and the UE during P2P communication, the solid line for data link between the two UEs, and the arrowhead for direction of information flow. It can be obviously seen from the figure that only signaling link exists between the UTRAN and the UE, while only data link exists between the two communicating UEs. If additional signal overhead for management is ignored, P2P communication can save about 50% radio resource during the process of direct link. Furthermore, control channels are reserved between the UTRAN and the UEs, so wireless network operators still holds control through the base station over how the UEs utilize radio resources.
It is commonly accepted that a Time Division Duplex (TDD) air interface is a communication standard that offers a more flexible adaptation to different uplink and downlink traffic requirements. Among existing 3G systems based on TDD communication mode, TD-SCDMA (Time Division-Synchronization Code Division Multiple Access) system is the most suitable system for the combination of P2P communication with conventional communication mode, because the same carrier frequency is applied in both uplink and downlink communications, which can simplify the RF (Radio Frequency) module of the mobile terminal.
In a TD-SCDMA system that is capable of employing P2P communication mode, the DIRECT mode is introduced to describe the direct communication between two UEs, besides two other working modes—IDLE mode and CONNECT mode defined in conventional TD-SCDMA system. The communication link in direct mode can be defined as FORWARD link (e.g.: the link from UE1 to UE2) and BACKWARD link (e.g.: the link from UE2 to UE1) identified according to the information flow direction for one UE to send signals to the other UE or receive signals from the other UE. Because P2P communication mode is constructed in combination with existing TD-SCDMA systems, the UTRAN, the P2P UEs and other conventional UEs allocated in the same timeslot can overhear the information transferred on the FORWARD link or BACKWARD link, i.e.: from the view of the UTRAN, even though the UEs have no connection with the UTRAN, the FORWARD link and BACKWARD link are associated with a certain uplink timeslot or downlink timeslot (the FORWARD link and BACKWARD link can correspond to different uplink timeslot or downlink timeslot depending on different resource allocation schemes). Hence P2P communication will cause signal interference to conventional communication. Similarly, two P2P UEs can also overhear the information transferred in the uplink timeslot or downlink timeslot associated with its FORWARD link or BACKWARD link during P2P communication. Therefore, when conventional links share the same timeslots with the P2P links, conventional uplink or downlink communication will interfere with the communication of the P2P FORWARD link or BACKWARD link, which seriously deteriorates the performance of P2P-enabled TDD CDMA communication systems.
To improve the performance of P2P-enabled TDD CDMA communication systems, it's necessary to effectively reduce the signal interference caused by P2P communication mode to the TD-SCDMA communication systems. First of all, analysis will go to the interference signal brought by introducing P2P communication mode in the following, and then how to reduce interference signal will be described. For simplicity in the following, the timeslot in which one UE transmits signals to the other UE through the above FORWARD link or BACKWARD link is called transmit timeslot (Tx timeslot), while the timeslot in which the UE receives signals from said another UE through the above FORWARD link or BACKWARD link is called receive timeslot (Rx timeslot), wherein the Tx timeslot or Rx timeslot is associated with an uplink timeslot or downlink timeslot in the sub-frame in conventional communication respectively.
1. Interference Associated with Uplink Timeslot between P2P Link and Conventional Link
FIG. 3 illustrates the interferences between P2P link and conventional link in P2P-enabled TD-SCDMA systems when the P2P link is associated with uplink timeslot. As shown in FIG. 3, it is assumed that UE1 and UE2 work in P2P mode and UE3 works in conventional mode, wherein UE1's Tx timeslot is associated with UE3's uplink timeslot, that is, UE1 and UE3 are allocated in the same uplink timeslot to transmit signals respectively to UE2 and the UTRAN. S1 is the information from UE1 to UE2 through direct link (taken as FORWARD link) and S2 is uplink information from UE3 to the UTRAN through uplink, moreover, S1 and S2 are associated with the same uplink timeslot but with different spreading codes.
In TD-SCDMA communication systems, one of the most important features is to maintain uplink synchronization, which means signals from different UEs should arrive at the UTRAN at the same time to guarantee the orthogonality of the spreading codes of signals from the main paths of different UEs.
For conventional communication systems, the UTRAN monitors and controls the UEs' uplink transmitting timing via a specific traffic burst structure in CONNECT mode so as to maintain uplink synchronization for each UE. But for P2P communication mode, the UTRAN is only involved in P2P link establishment procedure and not involved in the P2P communication procedure after P2P link's establishment. Therefore, during P2P communication, there is no dedicated channel between the UTRAN and the two P2P UEs, so the UTRAN cannot adjust the uplink synchronization advance for the two P2P UEs transmitting signals by using specific traffic burst to maintain uplink synchronization even if it can overhear and estimate the uplink synchronization shift of the two P2P UEs.
Referring to FIG. 3, when UE1 and UE3 transmit signals in the same uplink timeslot, the UTRAN can overhear information S1 transferred from UE1 to UE2 (to the UTRAN, S1 is considered as interference signal 11). But as described above, there is no dedicated channel between the UTRAN and UE1, so the UTRAN can't adjust UE1's transmission timing by using the traffic burst in conventional communication mode even if it can overhear information S1 and estimate UEI's synchronization shift information, which means UE1 working in P2P mode may lose uplink synchronization with the UTRAN (UE3 working in conventional mode can maintain uplink synchronization with the UTRAN with conventional mode). That is, I1 and S2 are likely to reach the UTRAN unsynchronously, which will potentially impair uplink synchronization and thus degrade the system performance.
Similarly, when UE1 and UE3 transmit signals in the same allocated uplink timeslot, UE2 can also overhear signal S2 transferred from UE3 to the UTRAN (to UE2, S2 is considered as interference I2), and interference signal I2 will also produce impact on UE2's receiving S1, which may potentially impair the P2P communication quality.
2. Interference Associated with Downlink Timeslot between P2P Link and Conventional Link
FIG. 4 illustrates the interferences between P2P link and conventional link in a P2P-enabled TD-SCDMA system when the P2P link is associated with downlink timeslot. It is assumed that UE1 and UE2 work in P2P mode and UE3 works in conventional mode, wherein UE1's Rx timeslot is associated with UE3's downlink timeslot, that is: UE1 and UE3 are allocated in the same downlink timeslot to respectively receive signals from UE2 and the UTRAN. S3 is the P2P link information from UE2 to UE1 via direct link (taken as BACKWARD link) and S4 is downlink information from the UTRAN to UE3 via downlink, furthermore, S3 and S4 are associated with the same uplink timeslot but with different spreading codes.
In FIG. 4, the downlink information S4 transmitted from the UTRAN to UE3 may produce interference to other UEs who share the same downlink timeslot with UE3 but use different spreading codes to receive signals. Such interference is called multi-access interference (MAI).
Referring to FIG. 4, when UE1 and UE3 receive signals in the same allocated downlink timeslot, UEI can overhear information S4 transferred from the UTRAN to UE3 via downlink (to UE1, S4 is considered as interference signal I4), and generally the transmission power of signals from the UTRAN is relatively strong, so the interference signal I4 is likely to impair the direct communication quality seriously.
Similarly, when UE1 and UE3 receive signals in the same allocated downlink timeslot, UE3 can also overhear information S3 transferred from UE2 to UE1 (to UE3, S3 is considered as interference signal I3, and meanwhile UE2 can be taken as the pseudo-UTRAN of transmission information in downlink timeslot), and the interference signal I3 will impair the communication quality of UE3 near UE2 and other UEs in the same timeslot as UE3 to receive signals.
3. Interference between P2P Direct Link Pairs
FIG. 5 illustrates the interferences between two P2P direct link pairs in a P2P-enabled TD-SCDMA system, wherein a UE in one of the two P2P link pairs receive transmit signals to another UE in another P2P link pair. Assume that UE1 and UE2 work in one P2P link pair while UE3 and UE4 in another P2P link pair.
Because the P2P link pairs are symmetrical, in the associated timeslot, signal S5 or S6 from UE1 to UE2 will become interference I5 or I6 to UE4 who is receiving signals from UE3. Obviously these interferences may also greatly impair the direct communication quality.
As noted above, after P2P link is introduced in conventional TD-SCDMA systems, there exist 6 possible interference signals I1, I2, I3, I4, I5 and I6. Depending on whether the UTRAN is involved, the above 6 interference signals can be divided into two types. The first type includes interferences between the UEs, such as I2, I3, I5, and I6; and the second type includes the interferences with UTRAN involved, such as I1 and I4.
To guarantee the communication quality of a P2P-enable TD-SCDMA communication system, effective methods needs to be researched to cancel the above 6 interferences (it's better to achieve that without changing the physical layer structure of existing communication systems). Among the above 6 interference signals, the first type can be reduced or cancelled by efficiently limiting the radio range supported by P2P and adopting intelligent radio resource control scheme, while interference signal I1 can be cancelled as described in a patent application entitled “A Method and Apparatus for Uplink Synchronization Maintenance with P2P Communication in Wireless Communication Networks,” filed by KONINKLIJKE PHILIPS ELECTRONICS N. V., on Mar. 7, 2003, Attorney's Docket No. CN030004, application Ser. No. 0319894.5, the disclosures of which are hereby incorporated by reference. As for interference signal I4 of the second type, there is no effective solution yet now.
From the foregoing interference analysis, it can be seen that interference signal I4 is introduced by the UTRAN's transmitting signals to UE3 via the downlink to UE1 in the same downlink timeslot as UE3. Usually, the signal transmission power of the UTRAN is relatively strong enough that all UEs sharing the same downlink timeslot in the same cell can overhear the signal transmitted, and moreover the signal is the mixed one including redundant information of many other UEs, hence, I4 can't be ignored. In UE1, UE1 must adopt MUD (multi-user detection) or JD (joint detection) to cancel the interference signal, so as to guarantee the direct communication quality.